Polyolefin foam, such as low density polyethylene foam, is commonly made by combining a physical blowing agent with molten polyethylene resin under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
In the past, physical blowing agents widely used for making polyolefin foams were chlorofluorocarbons and hydrochlorofluorocarbons. Use of such blowing agents, however, has been or will be banned because of environmental concerns.
Presently, physical blowing agents more commonly used for making low density polyethylene (LDPE) foams are hydrocarbons such as isobutane or blends of isobutane and n-butane. Other hydrocarbons such as ethane and propane have been used more recently in making LDPE foams. The ability of isobutane, n-butane, propane, ethane and combinations thereof to give stable, low density foams depends on factors such as desirable solubility in low density polyethylene, and the ability of gas permeation modifiers to slow down the escape of such blowing agents. The resultant foam article (e.g., a sheet) using such blowing agents is frequently produced with at least some corrugation. Corrugation occurs when the radial rate of expansion is higher than the radial space available for the foam as it exits the die. Corrugation may be reduced to a certain extent by optimizing the foaming process and apparatus used in forming the foam with these blowing agents, but a low degree of corrugation or visible corrugation lanes often remain. The corrugation becomes more pronounced when a fluid with a very low boiling point (e.g., ethane or propane) is (a) used as the sole blowing agent or (b) present in an amount greater than about 5 mol % with a higher boiling fluid (e.g, isobutane). Corrugation also tends to occur more frequently in sheets (thickness of up to about ½ inch) as opposed to planks (thickness of greater than about an inch), and the degree and magnitude of corrugation increase as the foam density decreases.
Therefore, a need exists for a stable foam having minimized or no corrugation, and a process for making the same.